The present invention relates to a development material for wide use in the electrophotographic apparatus, electrostatic recording technique etc., and more particularly to a magnetic toner of the single component type for use, say, in the magnetic brush development process.
The conventional developer of the dry process consists of a coloring material called a toner which is mainly composed of synthetic resin and a separate carrier containing iron powder, etc. The development process employing such a development material comprises the frictional electrical charging of toner by the friction between the toner and the carrier and the activity of electrostatic attracting power between said toner and the electric charge of the surface of the drum or recording medium, thereby forming a fixed image on said recording medium and the like. Many processes for transporting the toner and attaching it to the electrostatic latent image are known, such as the cascade development process, fur-brush development process, powder cloud development process, impression development process and magnetic brush development process. The most important problems about such a toner of binary component system are that the toner used repeatedly for a long time is attached to the surface of the carrier, and that the distribution of charge in the toner mass becomes unequal by the pulverization of the carrier particles and other reasons, resulting the degeneration of the quality of image. Furthermore, the carrier must be exchanged and its maintenance is inconvenient.
Recently a processs more convenient than employing a magnetic roll was developed. This process features that the toner particles internally contain magnetic particles, referred to herein as single component toner, thereby dispensing with carrier particles. Furthermore in regard to the fixing process, the thermo setting and pressure setting processes, which are simpler than development using magnetic rolls have come in use. In the processes of thermosetting and pressure-setting, said magnetic toner is transported from its supply vessel by the magnetic power of a magnetic roll to the development zone; and the characteristics required of the magnetic toner in this step include the magnetic characteristics, such as the saturated magnetization 4.pi.Is, coersive force Hc and Curie point Tc, the electric characteristic of being attracted to the electrostatic latent image under the presence of electric charge or electric field and against the magnetic power and fluidity and the setting characteristics discussed below. The magnetic characteristic among these characteristics is substantially defined by the magnetic characteristic of the magnetic material itself and its content.
The requirements of the setting characteristic differ depending on the type of process. In thermosetting process, the requirement is rapid melting and fixing at a predetermined temperature; and in pressure setting process, the requirement is the fixability under extreme low pressure. In order to obtain sharp images without the blackening or coloring of background, all the toner particles should preferably have equal electric charge and particle size.
Examples of the pigments and dyestuffs available as antistatic agent and/or coloring agent include carbon black, nigrosine dyestuffs, aniline blue, chrome yellow, chalco-oil-blue, chinoline yellow, malachite green oxalate, lamp black, robe bengal, methylene blue chloride, DuPont oil red, other azo-compound, metal-containing dyestuffs, organic acid metal salts and the mixtures thereof. Said coloring materials etc., however, present generally very poor miscibility with adhesive resin and require melting and kneading for a long time in order to obtain an evenly distributed mixture. The distribution of the electric charge in toner is often uneven because of the fluctuation of composition. This fact is one reason for the blackening of background.
Furthermore, some of the widely known adhesive resins such as polyethylene, styreneacryl copolymer, polyamide and epoxy-vinyl-styrene, etc., contain functional radicals, and when these kinds of resin are employed to prepare a magnetic toner with excellent fixability and fluidity, the toner particles often coagulate or solidify because of the atmospheric humidity and/or the temperature rise within the facsimile apparatus, so that development may become impossible or fluidity may be lowered or the toner may be altered. In other words, the toner shows a poor preservability (heat resistance, moisture resistance, etc.). Such a degenerative phenomenon occurs particularly in summer and is particularly troublesome for the thermosetting toner which is difficult to preserve a long time. The degeneration of preservability results not only from the above-mentioned reason but also from the mechanical impact due to the mechanical friction with the sleeve covering the small gaps in the doctor plate of the development member and the magentic roll so that the surface layer of the toner and the particles themselves may be broken. These breaks also tend to cause the blackening of background on the electrostatic charge image.
In order to avoid the degeneration of the durability, the heat resistance and mechanical strength of the resin should be enhanced or, in other words, a high-molecular weight containing smaller number of terminal radicals should be used. In this case, however, the setting characteristic is generally degenerated and the pulverization to extreme fineness generally poses a great difficulty in the process of preparing toner.